JUNE DIVE GUIDE COLUMN: Slugs and Sharks

Column by Chris Gooda
Images by Elke Bojanowski

This season has brought a real treat to the southern marine parks of Egypt.  Silky Sharks (Carcharhinus falciformis) have not been regularly spotted in the Egyptian Red Sea for several years, but in the past couple of months these inquisitive predators have been thrilling divers with close encounters at dive sites as diverse as Brothers and the Thistlegorm!  Growing to almost 3.5 m, this requiem shark, listed by the IUCN as "near threatened" is one of the most abundant sharks in the pelagic zone in four distinct populations across the tropical waters of the globe.  Unfortunately they also make up a significant share of commercial shark fisheries around the globe.

Tracking studies in Eastern Pacific and Gulf of Mexico have shown that these sharks spend 99% within 50m of the surface (ideal form sightings while recreational diving) although are capable of diving to depths of 500m.  Silky sharks are pelagic, their preferred habitat is around islands and reefs with access to deep water, although they can venture into shallower coastal waters.  This is what we are currently observing in the Egyptian Red Sea.

One possible explanation for the sudden abundance of these creatures is the unusually high water temperatures that we experienced since the beginning of winter 2009.  In the Pacific Ocean silky sharks are known to prefer waters of 26-30°C and retreat to higher latitudes during El Nino events to seek cooler water.  Silky sharks are regularly spotted in Sudanese waters (where Blue Fin will be spending a large part of next year) and it could be that part of this population has temporarily headed North to more temperate waters.

During this season divers have regularly been able to experience these magnificent creatures directly under their dive boat at anywhere from 25m to the surface.  They have often been curious and inquisitive, providing a unique opportunity to observe the sharks' bulky bodys and shiny skin close-up.

Head-shield slugs have a made a late appearance in the Red Sea this year.  Spending most of the year living a few centimetres below the sand they are most often spotted on shallow sandy bottoms in large gatherings (especially during the night) in spring and early summer.  The most common is the Blue-spotted shield slug (Chelidonura livida) which is a very dark blue with flourescent-blue rings and spots.  It is unique to the Red Sea and Mauritius.

Some head-shield slugs still possess small external shells (too small for them to retract into) whilst others have internal shells.  The species found in the Red Sea have very sensitive eyes on the anterior of their head and cillia (small hairs) around the mouth which are used to track the mucous trail of prey and mates as well as to keep the mouth sand-free.  Head shield slugs are not nudibranchs (they have no visible external gills) but are molluscs in the sea slug family.

Column by Chris Gooda
Images by Elke Bojanowski

^ top